Within the popular narrative, the touching down of European diseases in the New World was seen as some sort of chance calamity: the Europeans happened to be exposed to certain diseases on their side of the pond, which they brought with them, subjecting the indigenous peoples to foreign elements they had not yet built up immunity to. But of course there was much more going on than that.
The organisms behind the European diseases, as living things, had co-evolved to take on the nature of those peoples, exploding in number and lines of evolution and driven to conquest. Something like a virus evolves in a certain context - and acts in a certain context. The context in this case was an explosion in world trade and the glimmerings of early industrialization.
There is always the matter of chance in nature, or at least the sort of chance that serves as placeholder for our own limited understanding. But there is also the extent to which human beings (at least certain societies of them), in their explosive expansion, have affected their environments, and how their affected environments have affected them.
A virus that must subsist deep in the forest is a very different beast from the kind that can ride the material waves of world-industrial flows, coming into contact with many other organisms and picking up the biological tools required to perpetuate and further their flourishing. And the human beings that have been subjected to that very different beast are very different beasts themselves.
